The present invention relates generally to electronic circuits and, more particularly, to an integrated circuit, low current reference generator.
Modern integrated circuits are often required to operate with very small values of supply current. When this value is in the microampere range, developing a practical integrated solution is a particular challenge due to the inherently large size and/or inaccuracy of any device that must withstand a large amount of supply voltage without passing significant amounts of current. For example, for a resistor to be required to limit a 10 volt supply to a 2 microampere (.mu.A) bias current, would necessitate a 5 Mfi resistance, a value which is impractical in the realm of most of today's integrated circuits.
In addition, it is often desired to be able to switch the circuit off and have the entire integrated circuit (IC) appear to the system as if it were an open circuit. For the convenience of the external implementation in a particular application envisioned for the present invention, it is desired to have this enabling function operate in the transistor-transistor logic (TTL) threshold specification, and enable the circuit to its powered-on condition when the voltage level of an interface signal is below the lower TTL threshold, i.e., 0.8 volts, referred to as active-low logic. This requires a switchable input that will use no current when it is off.
One solution to the low current reference is to use a bootstrapped current mirror, where the generated current is reflected back to the input to serve as a load. This can accurately provide very low levels of current. However, such schemes are plagued by start-up problems; they require some technique to get current flow started. A typical approach may be to use a capacitor attached to ground. Thus, when the supply turns on, the capacitor injects the stored charge into the current mirror and current flow is initiated, with the bootstrap arrangement forcing it into regulation. However, if such a circuit is disrupted during its normal operation, and the supply voltage remains stable, the mirror may be forced into a permanent "off" condition.
In view of the above, it is clear that there exists a need to develop an improved reference generator for providing the extremely low current required by modern integrated circuit devices over what is presently known in the art. Such a reference generator would ideally supply a bias current with very little temperature dependence and would provide high rejection of transients on the input voltage. It would use very little overhead current, and it would provide means for ensuring that it could not be forced permanently out of regulation by system transients. It would preferably avoid the use of the traditional capacitive trigger as the start-up circuit, so that the power supply would not have to be ramped each time the bias is lost during operation. Finally, such a reference generator would preferably have an active-low logic implementation with switching near the TTL range and with a zero-current off state.